September 5, 2008 6881076C25-E
4-72 Troubleshooting Procedures: Power Amplifier Procedures
4.5.3.1.4 Isolating Failures
Methods of analyzing individual stages of the Power Amplifiers are detailed below. Most of the
stages are Class C and must he analyzed under relatively high RF power levels. Generators capable
of such levels may not be available in all service shops, therefore the tests below are arranged in
order of increasing power. This tends to allow the preceding stage to be the source of RF power for
testing the next stage. If adequate power sources are available, then any stage may be tested with
external signal injection.
1. Testing Buffer Circuitry
The required DC and RF conditions are defined in Table 4-23 on page 4-67. With no RF input
applied, the collector voltage of Q9800 should be 9.4V. If not, check L9805, L9801, and the
feed runners. The base voltage should be 0.6-V (0.7-V without RF). If not, check R9801,
CR9800, and related adaptive bias circuitry.
To check for power out, remove R9805 and lift the output end of C9807. Solder the center
conductor of a small-diameter 50-ohm, coax to the vacated pad on the buffer side. Solder the
coax's shield to ground. Under the conditions specified in Table 4-23 on page 4-67, the
measured power should be at least 350 mW. After output power has been tested, replace the
resistor and capacitor with new parts.
An alternate method of testing the buffer's power out is to carefully lift the input lead of the
power module ( pin 1) from the circuit board and replace it with the center conductor of a
small-diameter coax. Solder the shield of the coax. Solder the shield of the coax to the
adjacent ground pad.
To test the input VSWR of the circuit, apply 70 mW to the input. Using a directional coupler,
verify that the reflected power is less than 20 mW.
2. Testing the Power Module (U9850)
The power module is a packaged gain block with 50-ohm input and output impedances. It has
three gain stages, the first two of which have controlled voltage applied (for regulating power)
and the final stage has A+ applied.
If the buffer stage has not been confirmed in "working order," an external 400 mW must be
injected. Do this by carefully lifting pin 1 of the power module and soldering the center
conductor of a small diameter coax to the pin. Solder the shield to the ground pad adjacent to
pin 1. To this cable, inject 400 mW. (This application is not so critical to require an 'N'
connector on the loose end of the coax.)
If the buffer stage is confirmed in "working order," then provide 100 mW drive to the buffer
(K9.4-V must be applied) to drive the module.
To measure the output power from the module, remove the series DC blocking capacitor
C9879 (15W) or C9856 (35W), then connect a 39 pF blocking capacitor from the center
conductor of a small diameter coax to the vacated pad, and finally, ground the shield of the
output coax. Use this coax to measure output power.
Control voltage (Pins 2 and 3) should be 10 V; A+ ( pin 4) should be 13.0 V. Apply voltages
through the DC connector on the PA board.
With either 100 mW applied to the buffer or 400 mW applied to the module input, the output
power should be at least 15 Watts. If power out is less than 15 Watts, the module is defective
and must be replaced.
NOTE: When replacing the module, apply thermal compound on the heatsink surface.
Torque the screws to the correct value; see the ASTRO Digital Spectra and Digital
Spectra Plus Mobile Radios Basic Service Manual (6881076C20).
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